Let's find out.
It can be easy to dismiss an aircraft flying overhead.
"On any given day, more than 87,000 flights are in the skies in the United States. Only one-third are commercial carriers, like American, United or Southwest. On an average day, air traffic controllers handle 28,537 commercial flights (major and regional airlines), 27,178 general aviation flights (private planes), 24,548 air taxi flights (planes for hire), 5,260 military flights and 2,148 air cargo flights (Federal Express, UPS, etc.). At any given moment, roughly 5,000 planes are in the skies above the United States. In one year, controllers handle an average of 64 million takeoffs and landings." - From the National Air Traffic Controllers Association
Believe it or not, it's easier to track these aircraft than one may initially think. The following visualization demonstrates the most common subset of aircraft that fly through the majority of Californian skies. The dataset was scraped from flightaware's (see spotting section below) awesome API, and averaged to represent a common spring day.
San Francisco
San Jose
Los Angeles
An average day overhead.